A senior leader and key financier of the al-Qaida-linked Haqqani network - one of the biggest threats to U.S. troops in Afghanistan - has been shot dead on the outskirts of Pakistan's capital.

Taliban sources and intelligence officials say gunmen killed Nasiruddin Haqqani late Sunday in a residential area of Islamabad and that his body was taken to the North Waziristan tribal region to be buried.

The Haqqani network - based in northwestern Pakistan - is an important ally of the Afghan Taliban and has pledged allegiance to its leader, Mullah Omar, though it operates independently.

The killing comes just more than a week after a U.S. drone strike killed Pakistani Taliban chief Hakimullah Mehsud in North Waziristan tribal district.

U.S. officials have accused Pakistan's intelligence agency of supporting the Haqqani network as a key proxy in the Afghan war - an allegation denied by Islamabad.

Washington holds the Haqqanis responsible for some of the most high-profile attacks in Afghanistan, including a 2011 siege of the U.S. embassy and, in 2009, the deadliest attack on the CIA in 25 years.

Nasiruddin Haqqani was considered an important financier and emissary for the network, which is currently led by his brother, Sirajuddin Haqqani. Their father, Jalaluddin Haqqani, founded the group and is well-known for fighting the Soviets after they invaded Afghanistan in 1979.

The United States put the Haqqani network on its terror blacklist in September 2012 and the Pentagon said the group represented a "significant threat" to national security.

The United Nations followed with global sanctions on the network two months later, saying in its designation that the group was linked to al-Qaida, the Islamic Movement of Uzbekistan and a string of militant groups in Pakistan including the TTP.